The present invention relates to a method in connection with the manufacturing of paper and cardboard. The method aims to level out quality variations and to raise the quality level through converting excess waste for manufacturing, so-called broke. The converted broke is returned in a more controlled way to the paper of cardboard, as compared to conventional techniques.
When manufacturing paper and cardboard from cellulose fibres, great emphasis is placed on the fibre composition for different products in order to utilise the fibres"" properties in the best way. So, for example, paper and cardboard are manufactured from different stock compositions consisting of different proportions of different fibre and pulp types depending on which properties are desired in the final product. Even if a given paper or cardboard quality has a given nominal fibre composition, there is a natural variation in the fibre raw materials included as different wood species have different fibre characteristics, see Table 1, and there are also natural property variations for the fibres in wood with regard to length, width, fibre wall thickness, etc. The fibre length distribution for a Swedish softwood pulp spans from fractions of millimeters up to 6-7 mm. For birch pulp, the corresponding value is from fractions of millimeters up to approx. 3-4 mm. This means that fresh cellulose pulps demonstrate major non-homogeneities in fibre property distributions.
When manufacturing paper and cardboard, there is always produced a quantity of excess material, so-called broke. This broke can comprise edge strips, widths on rolls of final product which results in that the whole machine width cannot be used to the full, second-rate quality, etc. From the mentioned examples of factors which result in broke, it is understood that the broke quantity varies over time. If one manufactures a product which complies with the quality specification and utilises the whole machine width, there will be small quantities of broke. When for some reason there are problems with complying to the quality specification, e.g. at a change of quality on the machine, and the full machine width cannot be used, the quantities of broke will become larger.
Table 2 shows a calculation example of how a varying broke quantity changes the fibre composition in a three-ply product for different broke mixes. The example is based on the broke being returned directly to the inner ply.
All paper and cardboard qualities contain larger or smaller nominal quantities of broke. Only in cases where the whole quantity of broke can be used directly, with or without subsequent processing, quality variations due to nominal fibre composition in the paper or cardboard product is avoided, i.e. if e.g. 15% of the production results in broke when a quality which nominally should contain 15% broke of the production is manufactured at the same time. In all other cases, the broke will contribute to a quality variation by a varying fibre composition which deviates from the nominal fibre composition, due to the fact that there is a surplus of broke which must be stored, a lack of broke, stored broke of a different fibre composition compared to the quality which is presently being manufactured, etc. Broke quantities varying over time will sooner or later result in quality variations for all paper and cardboard qualities.
According to known technique, the broke can be managed in different ways depending on the broke quantity. Conventionally, the disintegrated broke is recycled, either directly, after storage in a vat/tower or after storage as a roll, to the paper or an inner ply for multi-ply paper and multi-ply cardboard. In the ideal case, the broke is broken up in water, where appropriate subsequently processed through beating or refining, and re-used in the production directly together with the originally included cellulose pulps. The varying broke quantities will however often result in broke having to be stored. This storage can be performed in two ways. One way is that the disintegrated broke is stored in a storage vat/tower after the disintegration and possible subsequent processing. The other way is to have rolls of a outclassed quality in stock, which is disintegrated and where appropriate subsequently processed for use as needed.
In the case of multi-ply paper or cardboard, it is not only varying broke quantities that cause problems. By returning the broke to an inner ply in the cardboard, the inner ply to which the disintegrated broke is added will be brought to contain pulp with fibres that originate from an outer ply. The multi-ply paper or cardboard will thereby, calculated on its total weight, contain a larger proportion than nominally of fibres of the type which is intended to be used in an outer ply of the paper or cardboard and a smaller proportion than nominally of fibres of the type intended to be used in an inner ply of the paper of cardboard, see examples in Table 2.
It is known that the fibres in a fibre flow can be fractionated by means of a screen or hydrocyclone, a screen being used to fractionate fibres primarily according to fibre length, while a hydrocyclone is used to fractionate fibres with different thicknesses and thereby different fibre flexibilities. Studies have shown that with the aid of size fractionating (screen), it is possible to separate out a large share of short fibres from a fibre flow; Fredlund M. et al., xe2x80x9cFxc3x6rbxc3xa4ttrade kvalitetsegenskaper hos kartong genom fraktioneringxe2x80x9d, STFI-rapport TF 23, 1996, Stockholm, STFI; Grundstrxc3x6m K-J, xe2x80x9cSTFIs silteknik hxc3x6jer kvaliteten vid kommersiell driftxe2x80x9d, STFI Industrikontakt, 1995, no. 1, p. 7-8. It has also been documented that by using a hydrocyclone one can separate flexible fibres from more stiff fibres; Wood J. R. and Karnis A., xe2x80x9cDistribution of fibre specific surface of papermaking pulpsxe2x80x9d, PulpandPaper Canada 80 (1979):4, p. 73-78, Bliss T., xe2x80x9cSecondary fibre fractionation using centrifugal cleanersxe2x80x9d, Tappi Pulping Conference, 1984, 217 pp; Paavilainen L., xe2x80x9cThe possibility of fractionating softwood sulphate pulp according to cell wall thicknessxe2x80x9d, Appita 45 (1992):5, p. 319-326. In U.S. Pat No. 5,002,633 there is described a fractionating process which aims to separate the longest fibres from short fibres, fillers, contaminants, etc., from a pulp for re-use of the longest fibres in paper manufacturing.
Moreover, it is known to combine different fractionation equipment in fractionation systems for different purposes. In U.S. Pat No. 5,403,445, recycled fibres for manufacturing of paper with more than 70% recycled fibres are fractionated, and in U.S. Pat No. 5,061,345 a series of screens is used to separate out fibres from filler. In some fractionating systems, the aim is to separate fibres with different properties in order to be able to use the fibre fractions in different plies. This is described in U.S. Pat No. 5,147,505 where the fibres in a pulp are separated according to coarseness and the rougher fibres are used in one ply and the more slender fibres are used in another ply. In EP 0653516 A1, it is mentioned in a similar way that softwood fibres are separated into a fraction with thick-walled fibres which are used in one ply and a fraction with thin-walled fibres which are used in another ply.
In Vollmer H., xe2x80x9cSimulering av fraktioneringssystemxe2x80x9d, STFI Report TF 81, 1997, STFI, Stockholm, it is described how fractionators can be characterised for different operating conditions and how they distribute fibres with different properties in different fractions for given operating conditions. Thereby it is possible to predict the fibre property composition in the resulting fibre fractions when the operating conditions for combinations of given characterised fractionators are known, and when the fibre property composition of the input fibre flow is known.
It is also known per se to determine fibre characteristics on-line. Such systems are described e.g. in Fransson P-I., xe2x80x9cMxc3xa4tningar med STFI FiberMaster i ett kartongbrukxe2x80x9d, STFI Report TF 74, 1997, STFI, Stockholm; Karlsson H. et al., xe2x80x9cSTFI FiberMasterxe2x80x9d, STFI Report TF 70, 1997, STFL, Stockholm; Thomsson L. et al., xe2x80x9cUppskattning av andelen CTMP i centerskikt vid kartongtillverkningxe2x80x9d, STFI Report TF 78, 1997, Stockholm, STFI.
As a result of natural variations in the fibre raw material, varying broke quantity and varying broke composition, it is realised that it is impossible to completely avoid quality variations. Nevertheless, the variations constitute a problem in connection with increasing requirements on paper and cardboard qualities. At increasing requirements on efficient production and lower production costs, the need for optimum use of the fibre material is accentuated. This entails a need for all fibre material used for paper and cardboard manufacturing having to be used in the best way, i.e. the fibres should be used for that which they are best suited for. None of the above-mentioned documents discuss the problem of being able to manage broke for manufacturing of paper or cardboard and to be able to implement fractionation of this broke, whereupon the fractionation can be controlled so that different types of fibre in the broke can be conveyed to the most suitable ply/plies, in controlled proportions, in the paper or cardboard as produced.
By the present invention, a method in connection with the manufacturing of paper of cardboard is presented, whereupon excess material from the manufacturing, so-called broke, is re-used in an optimum way. Through the invention, the quality of the product may be controlled, the product""s service properties may be improved and quality variations in the paper or cardboard product may be levelled out.
These and other objects are achieved by means of the method according to the invention as described below.
According to one aspect of the invention, the fibre composition for the broke is determined by on-line characterisation of one or more of the fibre length, fibre width, fibre coarseness, fibre shape and fibre flexibility parameters, while the fibre composition in the fibre fraction(s) produced is determined in the same way or is calculated, and the fibre composition in the input paper pulp for the said given ply is determined by means of intermittent characterisation of one or more of the said parameters. Moreover, the fractionation is controlled on basis of one or more of the parameters fibre composition in the input paper pulp for the said given ply, fibre composition in the broke and fibre composition in at least one of the fibre fraction(s) produced.
According to another aspect of the invention the fractionation equipment used for the fractionation is characterised intermittently in terms of fractionating effect for different fibre compositions furnished to the fractionation equipment and operating conditions, whereby operating conditions refer to the input flow to the equipment, the ratio between input flow and reject, the concentration of the input flow, or similar operating conditions, which characterisation forms the basis for the said control of the fractionation. The fractionation is controlled, preferably continuously, by means of a change of at least one of the operating conditions of the fractionation equipment, which operating conditions include the input flow to the equipment, the ratio between input flow and reject, the concentration of the input flow, or similar operating conditions.
According to another aspect of the invention, the fractionation is performed in at least two steps, whereupon a first fraction is controlled primarily to contain short fibres, and another fraction is controlled primarily to contain long fibres. The fraction with long fibres is controlled through fractionation in a second step to consist of a second fraction primarily containing long flexible fibres and a third fraction primarily containing long stiff fibres, after which the said first and/or second and/or third fraction is distributed in a desired proportion to the said given ply or several given plies at the manufacturing the paper or the cardboard.
According to a further aspect of the invention, the fractioning is performed on basis of fibre length, preferably by use of a screen, while the fractionation on basis of fibre thickness and thereby fibre flexibility is performed preferably by use of a hydrocyclone. Using the method according to the invention, the broke in existing chemical short fibre pulp (preferably in the above-mentioned first fraction), chemical long fibre pulp (preferably in the above-mentioned second fraction which contains long flexible fibres) and mechanical pulp (preferably in the above-mentioned third fraction which contains long stiff fibres) may be returned in a desired proportion to the desired ply, which gives a higher and more even quality in the product, since despite varying broke quantity and broke composition it is possible to control the product to a nominal fibre composition in the ply/plies.
In the case of multi-ply paper or cardboard, using the invention, in the broke existing
chemical short fibres may be returned preferably to their original outer ply for which strict requirements for surface properties are imposed,
mechanical pulp may be returned preferably to its original inner ply for which requirements are imposed for filling,
chemical long fibres may be used optionally, after possible subsequent beating and/or fractionating, in an outer ply and/or as reinforcement in an inner ply. If the fraction containing mainly chemical long fibres undergoes further fractionation, the fine fraction can be conveyed to an outer ply and the coarse fraction can be conveyed to an inner ply as reinforcement, after beating.
The advantage of the method according to the invention is that by separating the different fibre components in the broke, a desired proportion of the fibre component can be controlled to be included in a certain ply, in a specific proportion, in the final product. In particular, on-line fibre characterisation, characterisation of fractionators and calculation of the fibre property composition of different fractions makes it possible to utilise fractionation in order to, with the aid of suitable combinations of fractionation equipment, achieve a very good possibility for optimum control of the fractionation at every individual step. Several subsequent fractionation steps may together create a fractionation system in order to tailor-make fractions with the desired fibre property composition. The desired fibre property composition in a certain fraction may thereafter be controlled in a desired proportion, on a par with the nominal fibre property composition, to be included in a desired ply. The product, i.e. the paper or cardboard, will thereby achieve a good quality and evenness in this good quality, despite the broke being included in the process.